Spectrum scarcity is a problem that has been observed in regulative frequency allocation charts for some time. All potentially interesting spectrum bands for mobile communication are already allocated to services. However, additional spectrum for mobile broadband is needed to cope with the exponential take-off of mobile broadband traffic. At the same time traditional spectrum regulatory methods are sometimes perceived as too slow to adapt to the sometimes rapidly changing economic and technical requirements, implying that large parts of the electromagnetic spectrum is licensed but not effectively used.
In particular, the TV broadcast spectrum is not efficiently used due to the way the TV broadcast networks have been deployed. They are based on the concept of high transmit towers with high transmit power serving large areas with digital or analog TV. This type of deployment makes the frequency reuse distance large—in the order of 100 km—implying a spatially sparse use of the frequency band. The geographical areas where a TV frequency channel is not in use have been termed TV white space for that channel.
Motivated by the underutilization of e.g. the TV broadcast bands, the research community has during the last decade performed research into so called secondary spectrum access. The goal of secondary spectrum access is to use licensed but unused parts of the spectrum, e.g. the TV broadcast bands, for communication in such a way that a primary user, i.e., the user of the service provided by the license holder or the one having prioritized right to use the spectrum, is not negatively affected by the transmissions in the secondary system.
The central idea behind secondary spectrum access by secondary systems is thus to use already licensed or allocated spectrum for secondary purposes, i.e., for communication between a secondary transmitter and a secondary receiver or two secondary transceivers. As an example, the TV white spaces in the TV broadcast spectrum could be used for secondary purposes. Secondary users in a secondary system may also be referred to as a white space devices (WSD) in a white space system, which are thus units that opportunistically use spectrum licensed or dedicated for a primary service on a secondary basis at times or locations where a primary user is not using the spectrum. As already mentioned above, the WSD or white space systems are not allowed to cause harmful interference to the primary service. Furthermore, the WSD and white space systems are not protected from interference from any primary service or user.
Recently, the United States (US) regulatory body Federal Communications Commission (FCC) has opened up the opportunity for secondary usage of the TV broadcast band in the US under a set of conditions. Furthermore, the regulator authority Ofcom is well on the progress of finalizing a rule set that allows secondary usage of the TV broadcast bands in the United Kingdom (UK). In Europe, the regulatory standardization group European Conference of Postal and Telecommunications Administrations (CEPT) SE43 has lately finalized a report outlining the requirements for operating as a secondary user in the TV white spaces. Thus, the process of opening up TV white spaces for secondary usage around the globe is well under way.
One commonality to the rules in place in the US and the proposed rules in Europe and UK is that one allowed way of discovering spectrum opportunities for secondary usage to get access to the TV white spaces, i.e., perform secondary transmissions in the TV bands, is to access a centrally managed database referred to as a geo-location database. Upon a query from a secondary user or a WSD, the geo-location database provides the WSD with a list of TV channels available for secondary usage, also called TV white space channels, at the location of the WSD. The WSD may provide information regarding its location and possibly also additional information in the database query. Furthermore, in the CEPT SE43 proposal, the WSD obtains maximum allowed transmit power levels associated with the channels available for secondary usage in the response from the database. These transmit power levels are based on an estimation of how much interference that would be generated in a worst case, including a margin to take into account the aggregated interference from multiple WSDs using the same white space spectrum.
The control of the aggregated interference towards a certain point, line, area or volume is an important problem since regulators enforce limits on the interference a system is allowed to cause to other systems. Particularly for secondary usage of spectrum, e.g., TV broadcast bands, it is of vital importance to assure that the interference caused by multiple secondary users or systems to a primary user does not exceed a threshold of harmful interference, or at least exceeds the threshold only with a low enough probability.
Setting an arbitrary margin to take aggregated interference from multiple WSDs into account is not the most efficient way of using the white space spectrum. A fixed margin cannot account for the different types of interference which is caused from different numbers of secondary users with different fading situations. There is a risk of choosing a margin value which is either overprotective, which would mean that the WSDs are not allowed to use spectrum which could actually be used, or which does not protect enough, which would mean that the WSDs would cause harmful interference in many cases.